Coating metal articles by dipping in bath of dissimilar molten metal



June 1953 DE YARMAN WALLACE ETAL 2,642,368

COATING METAL ARTICLES BY DIPPING IN BATH OF DISSIMILAR MOLTEN METAL Filed Jan. 19, 1950 INVE NTORS 0e Q/"ma/7%//ace axm'Le/andRManse/l Patented June 16, .1953

COATING METAL ARTICLES BY DIPPING IN BATH OF DISSIMILAR MOLTEN METAL De Yarman Wallace, Canfield, and Leland R. Mansell, Poland, Ohio Application January 19, 1950, Serial No. 139,524

fi'Claims. (01. 117-114) This invention relates to coating metal articles by dipping in a bath of dissimilar molten metal. More particularly it relates to the art of applying to metal articles a coating of a dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form, which bath is maintained'in a receptacle of metal different from the metal of the bath.

In its practical aspects the invention has to do broadly with the metal coating of metal articles by dipping into a bath of molten metal dissimilar to the metal of the articles, as, for example, in coating ferrous metal articles with spelter, tin, terne metal, aluminum, etc. Purely for purposes of explanation and illustration the invention will be described in connection with the hot dip galvanizing of ferrous metal articles and particularly steel pipe. I

In hot dip galvanizing and other hot dip coating processes the molten bath of coating metal is maintained in a receptacle of metal having a melting point higher than the melting point of the metal of the bath. For example, galvanizing pots are customarily made of ferrous metal while the spelter consists essentially of molten zinc whose melting point is materially lower than the melting point of the ferrous metal of the receptacle.

It is and has been for many years common knowledge among those skilled in the art that the life of a ferrous metal galvanizing pot is undesirably limited due to the eating away of the ferrous metal of the pot at the inner surface thereof where the molten spelter is in contact with such inner surface. The pot quickly becomes pitted on the interior and the pits rapidly enlarge and deepen until the pot either springs a leak or becomes so thin in places as to be unsuited, for further use. When the pot either springs a leak or becomes so thin in places as to be unsuited for further use it has been customary to remove the molten spelter from the pot and build up the thin or pitted portions by the deposition of weld metal. This procedure is undesirable, time wasting and ex pensive. Nevertheless prior to our invention no one has been able to materially decrease the rate of deterioration of galvanizing and other metal coating pots.

Moreover, hot dip coating processes have been attended by excessive dross formation. Indeed, formation of dross is a function of pot'deterioration as the metal which is removed from the inner surface of the pot combines with some I of the metal of the bath to form dross which is essentially an alloy of the'two metals. The more rapid the deterioration of the pot, the more rapid is the formation of dross. The dross settles to the bottom of the pot and has to be scraped out at great expense. The value of the dross is low as compared with the value of the coating metal. Not only is dross formation a function of pot deterioration but loss of valuable coating metal is likewise a function of pot deterioration because of the fact that coating metal combines with pot metalto form the dross. Although the problem of excess dross formation has long been appreciated and efforts have been made to reduce dross formation, no substantial advance has been made in that direction and it is still necessary in modern commercial hot dip coating processes to scrape out and remove very substantial quantities of dross with its valuable constituents of pot metal and coating metal.

We have discovered that when a bath of molten metal is maintained in a metal receptacle of metal different from the metal of the bath a direct current of electricity flows between the metal of the bath and the metal of the receptacle as a natural phenomenon. We have found that it is that flow of a direct current of electricity between the metal of the bath and the metal of the receptacle which is the primary factor bringing about pot deterioration and dross formation.

It has, of course, been known that galvanic action occurs when dissimilar metals are immersed in an electrolyte, as, for example, in storage batteries, electroplating mechanism, etc. However, so far as we are aware it has not heretofore been known that a flow of a direct current of electricity occurs as a natural phenomenon when one metal in molten form is disposed in a receptacle of another metal. natural phenomenon is evidenced bya small though substantial and measurable current flow.

The current is direct current and may flow either from the metal of the receptacle to the metal of the bath or in the reverse direction depending on therelative positions of the metals in the electromotive force series. When the current flows from the metal of the bath to the metal of the receptacle the dross forms as an alloy layer on the interior of the receptacle from which it readily sloughs off and falls to the bottom of the receptacle. When the current flow is from the metal of the receptacle to the metal of the bath the dross forms'in' the'bath and gravitates to the bottom. I

That

We have conceived that if the natural current flow between the metal of the pot and the metal of the bath in a hot dip coating process is counteracted or inhibited pot deterioration and dross formation will be greatly reduced; and we have demonstrated that it is possible and feasible to inhibit or counteract such natural current flow and that the result is great reduction of the rate of pot deterioration and dross formation. By so doing we greatly increase the life of the receptacle, reduce the loss of material in dross and reduce the labor expense incident to cleaning out the dross. We prefer to inhibit the natural current flow between the metal of the receptacle and the metal of the bath by connecting between the metal of the receptacle and the metal of the bath a source of direct current opposite in direction to the natural current. Desirably the potential of the direct current of electricity from said source is regulated so that there is no substantial flow of current in either direction between the metal of the bath and the metal of the receptacle as we find that when that is done optimum results are obtained, to wit, the rate of pot deterioration and the rate of dross formation are reduced to the minimum.

,It may under certain conditions be desirable to regulate the potential of the direct current of electricity from said source to impress current so that there will be a flow of small potential in the direction opposite the direction of the natural current fiow to inhibit local current flow in the bath.

We desirably provide a source of direct cur- .rent of electricity, as, for example, a rectifier wherein alternating current is changed into direct current, and connections from the output terminals of the rectifier respectively to the metal of the bath and the metal of the receptacle so that the direct current from the rectifier tends to flow in the direction opposite the direction of the natural current. Means such as a rheostat may be provided in the circuit for regulation of the current potential. A voltmeter is also preferably incorporated in the circuit to indicate the current potential therein. The rheostat is desirably adjusted while watching the voltmeter, preferably to a position in which the voltmeter reading is substantially zero, indicating no substantial current flow between the metal of the receptacle and the metal of the bath in either direction.

There is ordinarily provided mechanism for dipping the articles into the bath having a portion extending into the bath. Such mechanism is ordinarily of electroconductive material such as steel. The mechanism for dipping the articles is preferably mounted upon means electrically insulated from the receptacle. One of the leads from the source of direct current may be electrically connected with the dipping mechanism while the other lead is electricallyconnected with the receptacle. Desirably a plurality of leads extend from one of the terminals of the currentsource to the receptacle, being connected with the receptacle at points spaced thereabout to provide for distribution of the current about the receptacle and uniformly eflicient operation.

The receptacle may have an electroconductive band thereabout to assist in distributing the current throughout the periphery of the receptacle and one or more leads may extend from one of the terminalsof the current source to the band. The molten metal in the receptacle may be heated by a plurality of burners- It is difficul 9 adjust all of the burners uniformly with the result that the bath temperature varies across the receptacle. Since natural current flow is a function of temperature the temperature variation across the receptacle results in local current flow and variation in current flow between the receptacle and the bath at different points. The molten metal may be positive to the receptacle at one point and negative to the receptacle at another point. The resistance to current flow of the metal of the receptacle may be great enough to prevent proper current distribution about the receptacle to effectively counteract local current flow and the electroconductive band promotes uniform current flow about the receptacle and between'the various localized areas of the receptacle and the bath.

Other details, objects and advantages of the invention will become apparent as the following description of a present preferred embodiment thereof and a present preferred method of practicing the same proceeds.

In the accompanying drawing we have shown a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same. The single figure is a somewhat diagrammatic vertical cross-sectional view of apparatus for applying to metal articles a coating of dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form.

Referring now more particularly to the drawing, there isprovided a pot or receptacle designated generally by reference numeral 2 which comprises a generally vertical sidewall structure 3 and a bottom 4 connected with the sidewall structure in any suitable manner. as, for example, by welding 5. The receptacle 2 is preferably made of ferrous metal, as, for example, steel. It is desirably mounted so as to be electrically insulated from the earth although the specific method of mounting of the receptacle may vary depending upon the installation. Heat is applied to the receptacle by any suitable means (not shown) as well known to those skilled in the art.

Mounted within the receptacle 2 is a guide structure designated generally by reference numeral 6 and which forms no part of our invention. We preferably provide a mounting for the guide structure 6 which is electrically insulated from the receptacle 2. Disposed generally within the guide structure 6 is mechanism for dipping articles into a bath of molten metal, for example, spelter, contained within the receptacle 2, said mechanism being designated generally by reference numeral 1; The spelter preferably substantially fills the receptacle 2 to a level within, say, two or three inches of the top thereof. Consequently, the greater portion of the mechanism 1 is disposed below the level of the molten spelter. The mechanism 1 is mounted from above separately from the receptacle 2, the mounting for the mechanism 1 being preferably electrically insulated from the receptacle. The mechanism 1 may be of conventional construction and cooperates with the guide structure 6 to move articles, for example, steel pipes, into, through and out of the bath of spelter. The mechanism 1 as shown consists of a flight conveyor 8 conveyingpipes 9 one-by one downwardly at the right-hand side of the drawing and thence to the left and upwardly along the guide structure 6, the individual pipes rolling down a surface in of the guide structure.

ceptacle 2 and the metal of the spelter bath in the receptacle. In the case of molten spelter in a steel receptacle current tends to flow from the spelter to the receptacle. We provide a source of direct current of electricity, said source being shown diagrammatically at I2. It may, as above mentioned, be a rectifier, or it may be any other suitable source of direct current. The positive terminal of the source I 2 is electricallyconnected with the receptacle 2 by a lead or leads l3, Desirably a band M of electroconductive material, such as copper, is brazed onto the outside of the receptacle 2 and the lead or leads I3 extend to the band I i. If a plurality of leads l3 are employed, as is desirable, they extend from the positive terminal of the source I2 to the band It at points spaced about the receptacle 2. For example, we may employ four leads I 3 extending from the positive terminal of the source I2 to the mid-points of the sides and ends respectively of the receptacle 2. p Y

The negative terminal of the source I2 isconnected with the bath of molten spelter in the receptacle 2. Since the mechanism I is of electroconductive material the connection between the negative terminal of the source I2 and the bath of molten spelter can be effected by a lead I5 from the negative terminal to the mechanism 1, which mechanism serves to complete the circuit to the spelter bath. A rheostat I6 is disposed in the lead l5. A lead l8 having a voltmeter II therein extends between the band I4 and the lead I5 at a point between the rheostat I6 and the mechanism 1. The voltmeter may be a millivoltmeter which is best suited to measuring small current potentials such as are involved.

In use of our apparatus we preferably adjust the rheostat I6 while watching the voltmeter I'I until the needle of the voltmeter points directly at the zero mark or indicates a small current flow in the direction opposite the direction of natural current flow. Current from the source I2 opposes the now of current which nature tends to cause as above mentioned. The current input to the circuit from the source I2 is opposite in direction and in potential preferably substantially equal to or slightly greater than the current naturally occurring in the apparatus.

Our invention is applicable to any coating process in which metal articles are dipped into a bath of dissimilar metal in. molten form, which bath is maintained in a receptacle of metal dif, ferent from the metal of the bath. vWhile the articles shown as being coated are pipes any other articles may be treated in similar manner. The bath may be of any molten coating metal and the receptacle may be of any metal .suitable for holding the molten bath.

While we have shown and described a present preferred embodiment of the invention and have illustrated a present preferred method of practicing the same it is to be distinctly understood that the invention is not limited thereto but may be otherwise variously embodied and practiced within the scope of the following claims.

attests mainly of an alloy of the two metals and causedeterioration of the receptacle and contamination of the bath, the step of counteracting such de terioration and contamination by setting up between =themetal of the bath and the metal of the receptacle 9. counter potential of directcurrent electricity inhibiting thev tendency of said direct current of electricity to fiow between the metal of the bath and the metal of the receptacle.

2. In the art of applying to metal articles a coating of a dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form, which bath is maintained in a receptacle of metal different from the metal of the bath, in which bath a direct current of electricity tends to flow between the metal of the bath and the metal of the receptacle as a natural phenomenon tending to result in formation of dross consisting mainly of an alloy of the two metals and cause deterioration of the receptacle and contamination of the bath, the step of counteracting such deterioration and contamination by setting up between the metal of the bath and the metal of the receptacle a counter potential of direct current electricity overcoming the tendency of said direct current of electricity to flow between the metal of the bath and the metal of the receptacle.

and causing a direct current of electricity of small potential to flow in the direction opposite the direction in which said first mentioned direct current of electricity tends to flow.

3. In the art of applying to metal articles a coating of a dissimilar metal by dipping the articlesinto a bath of the dissimilar metal in molten' form, which bath is maintained in a receptacle of metal different from the metal of the bath, in which bath a direct current of electricity tends to flow between the metal of the bath and the metal of the receptacle as a'natural phenomenon tending to result in formation of dross consisting mainly of an alloy of the two metals and cause deterioration of the receptacle and contamination of the bath, the step of counteracting such deterioration and contamination by connecting with the 'metal of the bath and the metal of the receptacle a source of direct current of electricity 1 of polarity reverse to that of said first mentioned direct current of electricity inhibiting the tendency of said first mentioned direct current of electricity to flow between the metal of the bath and the metal of the receptacle.

4. In the art of applying tometal articles a coating of a dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form, which bath is maintained in a receptacle of metal different from the metal of the bath, in which bath a direct current of electricity tends to flow from the metal of the bath to the metal of the receptacle as a natural phenomenon tending to result in formation on the surface of the receptacle of dross consisting mainly of an alloy of the two metals, which dross sloughs off of the receptacle during use, and cause deterioration of the receptacle and contamination of the bath, the step of counteracting such deteriora.

7. tion and contamination by connecting with the metal of the bath and the metal of the'receptacle a source of direct current of electricity tending to flow from the metal of the receptacle to the metal of the bath inhibiting the tendency of said first mentioned direct current of electricity to flow from the metal of the bath to the metal of the receptacle.

5. In the art of applyin to metal articles a coating of a dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form, which bath is maintained in a receptacle of metal difierent from the metal of the bath, in which bath a direct current of elec-' tricity tends to flow from the metal of the receptacle to the metal of the bath as a natural phenomenon tending to result in formation in the bath of dross consisting mainly of an alloy of the two metals and cause deterioration of the receptacle and contamination of the bath, the step of counteracting such deterioration and contamination by connecting with the metal of the bath and the metal of the receptacle a source of direct current of electricity tending to flow from the metal of the bath to the metal of the receptacle inhibiting the tendency of said first mentioned direct current of electricity to flow from the metal of the receptacle to the metal of the bath.

6. In the art of applying to metal articles a coating of a dissimilar metal by dipping the articles into a bath of the dissimilar metal in molten form, which bath is maintained in a receptacle of metal diiierent from the metal of the bath, in which bath a direct current of electricity tends to flow between the metal of the bath and the metal of the receptacle as a natural phenomenon tending to result in formation of dress consisting mainly of an alloy of the two metals and cause deterioration of the receptacle and contamination of the bath, the steps of connecting with the metal of the bath and the metal of the receptacle a source of direct current of electricity opposite in direction to said first mentioned direct current of electricity and regulating the potential of the direct current of electricity from said source so that flow of current in either direction between the metal of the bath and the metal of the receptacle is inhibited whereby to counteract said deterioration of the receptacle and contamination of the bath.

DE YARMAN WALLACE. LELAND R. lflANSELLi References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 781,230 Rodinan Jan. 11, 1905 1,654,910 Barbier Jan. 3, 1928 2,216,519 Quarnstrom Oct. 1, 1940 

1. IN THE ART OF APPLYING TO METAL ARTICLES A COATING OF A DISSIMILAR METAL BY DIPPING THE ARTICLES INTO A BATH OF THE DISSIMILAR METAL IN MOLTEN FORM, WHICH BATH IS MAINTAINED IN A RECEPTACLE OF METAL DIFFERENT FROM THE METAL OF THE BATH, IN WHICH BATH A DIRECT CURRENT OF ELECTRICITY TENDS TO FLOW BETWEEN THEMETAL OF THE BATH AND THE METAL OF THE RECEPTACLE AS A NATURAL PHENOMENON TENDING TO RESULT IN FORMATION OF DROSS CONSISTING MAINLY OF AN ALLOY OF THE TWO METALS AND CAUSE DETERIORATION OF THE RECEPTACLE AND CONTAMINATION OF THE BATH, THE STEP OF COUNTERACTING SUCH DETERIORATION AND CONTAMINATION BY SETTING UP BE- 